He continued to play in ITF and ATP Challenger tournaments, garnering considerable success on the doubles circuit where he won multiple titles during the period.

He represented Denmark at the Hopman Cup that year, playing a few keenly contested matches against top-ranked opponents including Tomas Berdych and Mardy Fish, while also winning in doubles with Caroline Wozniacki.

Nielsen entered the 2012 Wimbledon Championships with Jonathan Marray as wildcard entrant, but the duo went on to win the tournament upstaging such higher-ranked opponents as the Bryan brothers en route to the title.

He continued to play on the Challenger tour in singles, while enjoying success in doubles on the main ATP circuit, winning again at the 2019 BMW Open.

Nielsen represented Denmark at the 2012 Hopman Cup alongside Caroline Wozniacki, but the pair could not get past the Group Stages.

He then played at the Australian Open and qualified for the main draw at a Grand Slam for the first time but lost to Kevin Anderson 6–1, 6–2, 6–4 in the first round.

[5] Playing doubles with Marray in June, their first pairing of the year, they reached the Nottingham Challenger finals, losing out to Treat Huey and Dominic Inglot in three tight sets 4–6, 7–6(9), 8–10.

Then Nielsen and Marray received a wild card at the All England Club and began their 2012 Wimbledon Championships with a 5-set win over Marcel Granollers and Marc Lopez, one of 4 teams they beat en route that qualified for Barclays ATP World Tour Finals.

At the start of 2013, Nielsen partnered Johan Brunstrom for the first time since September 2012 at Metz, they reached the final of 2013 Heineken Open at Auckland losing out to Colin Fleming and Bruno Soares in two tough sets 6–7(1), 6–7(2).

Played with André Sá in 2013 Power Horse Cup at Düsseldorf and fell in the Semis to Andre Begemann and Martin Emmrich.

Nielsen partnered Johan Brunstrom and they entered the 2014 Aircel Chennai Open in January, the pair did not lose a single set on their route to the finals and went on to win the title defeating Marin Draganja and Mate Pavic 6–2, 4–6, 10–7.

In January at Copenhagen during the Davis Cup Europe/Africa Zone Group II First Round Cyprus tie, he won both his singles and doubles matches partnering Thomas Kromann.

He then partnered Igor Sijsling for the 2014 Heineken Open and reached the quarters to only go down to the top seeds Alexander Peya and Bruno Soares 5–7, 5–7.

He played at the Australian Open alongside Brunstrom and reached the second round before going down to Julien Benneteau and Edouard Roger-Vasselin in a tight three-setter 6–0, 5–7, 3–6.